Strengthened glass with ultra deep depth of compression

ABSTRACT

Chemically strengthened glass articles having at least one deep compressive layer extending from a surface of the article to a depth of compression DOC of at least about 125 μm within the glass article. The compressive stress profile includes a single linear segment or portion extending from the surface to the depth of compression DOC. Alternatively, the compressive stress profile may include an additional portion extending from the surface to a relatively shallow depth and the linear portion extending from the shallow depth to the depth of compression.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application and claims the benefit ofpriority under 35 U.S.C. § 120 of U.S. application Ser. No. 16/182,004filed on Nov. 6, 2018, which is a continuation of U.S. application Ser.No. 14/926,425 filed on Oct. 29, 2015, now patent Ser. No. 10/150,698granted Dec. 11, 2018, which claims the benefit of priority under 35U.S.C. § 119 of U.S. Provisional Application Ser. No. 62/073,252 filedon Oct. 31, 2014, the contents of each of which are relied upon andincorporated herein by reference in their entireties.

BACKGROUND

The disclosure relates to a chemically strengthened glass article. Moreparticularly, the disclosure relates to chemically strengthened glasseshaving a deep compressive surface layer.

Strengthened glasses are widely used in electronic devices as coverplates or windows for portable or mobile electronic communication andentertainment devices, such as cellular phones, smart phones, tablets,video players, information terminal (IT) devices, laptop computers andthe like, as well as in other applications. As strengthened glasses areincreasingly being utilized, it has become more important to developstrengthened glass materials having improved survivability, especiallywhen subjected to tensile stresses and/or relatively deep flaws causedby contact with hard/sharp surfaces.

SUMMARY

Chemically strengthened glass articles having at least one deepcompressive layer extending from a surface of the article to a depth ofcompression DOC of at least about 125 μm within the article areprovided. In one embodiment, the compressive stress profile includes asingle linear segment or portion extending from the surface to the depthof compression DOC. Alternatively, the compressive stress profile mayinclude an additional portion extending from the surface to a relativelyshallow depth and the linear portion extending from the shallow depth tothe depth of compression.

Accordingly, one aspect of the disclosure is to provide a glass articlehaving a thickness t and a compressive region under a compressive stressCS_(s) of at least about 120 MPa at a surface of the glass article. Thecompressive region extends from the surface to a depth of compressionDOC, wherein 0.1·t≤DOC≤0.25·t, and has a compressive stress profile. Thecompressive stress profile has a first portion a extending from thesurface to a depth d_(a) and a slope m_(a), wherein the depth d_(a) isequal to the depth of compression and −0.4 MPa/μm≥m_(a)≥−3.0 MPa/μm. Insome embodiments, the portion a is linear or substantially linear.

Another aspect of the disclosure is to provide an alkali aluminosilicateglass comprising at least about 4 mol % P₂O₅ and from 0 mol % to about 4mol % B₂O₃, wherein 1.3<[(P₂O₅+R₂O)/M₂O₃]≤2.3, where M₂O₃=Al₂O₃+B₂O₃,and R₂O is the sum of monovalent cation oxides present in the alkalialuminosilicate glass. The alkali aluminosilicate glass is ion exchangedand has thickness t and a compressive region. The compressive region hasa compressive stress CS_(s) in a range from about 100 MPa to about 400MPa at a surface of the glass, and extends from the surface to a depthof compression DOC, wherein 0.1·t≤DOC≤0.25·t. The compressive region hasa compressive stress profile. The compressive stress profile has aportion a extending from the surface to a depth d_(a) and a slope m_(a),wherein the depth d_(a) is equal to the depth of compression DOC and−0.4 MPa/μm≥m_(a)≥−3.0 MPa/μm. In some embodiments, the portion a islinear or substantially linear.

Yet another aspect of the disclosure is to provide a glass articlehaving a thickness t and a compressive region. The compressive regionhas a compressive stress CS_(s) in a range from about 400 MPa to about1200 MPa at a surface of the glass article, and extends from the surfaceto a depth of compression DOC, wherein 0.1·t≤DOC≤0.25·t. The compressiveregion has a compressive stress profile, the compressive stress profilecomprising: a first portion b extending from the surface to a depthd_(b) below the surface and having a slope m_(b), wherein −40MPa/μm≥m_(b)−200 MPa/μm; and a second substantially linear portion cextending from about d_(c) to the depth of compression DOC and having aslope m_(c), wherein −0.4 MPa/μm≥m_(c)≥−3.0 MPa/μm.

These and other aspects, advantages, and salient features will becomeapparent from the following detailed description, the accompanyingdrawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a chemically strengthenedglass article;

FIG. 2 is a schematic representation of a compressive stress profileobtained by a single step ion exchange process;

FIG. 3 is a graphical representation of a photograph showingstrengthened glass articles 1) exhibiting frangible behavior uponfragmentation; and 2) exhibiting non-frangible behavior uponfragmentation;

FIG. 4a is a graphical representation of a photograph showingstrengthened glass articles 1) exhibiting frangible behavior uponfragmentation; and 2) exhibiting non-frangible behavior uponfragmentation;

FIG. 4b is a graphical representation of a photograph showingstrengthened glass sheets that exhibit non-frangible behavior uponfragmentation;

FIG. 5a is a schematic cross-sectional view of an embodiment of theapparatus that is used to perform the inverted ball on sandpaper (IBoS)test described in the present disclosure;

FIG. 5b is a schematic cross-sectional representation of the dominantmechanism for failure due to damage introduction plus bending thattypically occurs in strengthened glass articles that are used in mobileor hand held electronic devices;

FIG. 5c is a schematic cross-sectional representation of the dominantmechanism for failure due to damage introduction plus bending thattypically occurs in strengthened glass articles that are used in mobileor hand held electronic devices;

FIG. 5d is a flow chart for a method of conducting the IBoS test in theapparatus described herein; and

FIG. 6 is a schematic cross-sectional view of a ring on ring apparatus.

DETAILED DESCRIPTION

In the following description, like reference characters designate likeor corresponding parts throughout the several views shown in thefigures. It is also understood that, unless otherwise specified, termssuch as “top,” “bottom,” “outward,” “inward,” and the like are words ofconvenience and are not to be construed as limiting terms. In addition,whenever a group is described as comprising at least one of a group ofelements and combinations thereof, it is understood that the group maycomprise, consist essentially of, or consist of any number of thoseelements recited, either individually or in combination with each other.Similarly, whenever a group is described as consisting of at least oneof a group of elements or combinations thereof, it is understood thatthe group may consist of any number of those elements recited, eitherindividually or in combination with each other. Unless otherwisespecified, a range of values, when recited, includes both the upper andlower limits of the range as well as any ranges therebetween. As usedherein, the indefinite articles “a,” “an,” and the correspondingdefinite article “the” mean “at least one” or “one or more,” unlessotherwise specified. It also is understood that the various featuresdisclosed in the specification and the drawings can be used in any andall combinations.

As used herein, the terms “glass article” and “glass articles” are usedin their broadest sense to include any object made wholly or partly ofglass. Unless otherwise specified, all glass compositions are expressedin terms of mole percent (mol %) and all ion exchange bath compositionsare expressed in terms of weight percent (wt %).

It is noted that the terms “substantially” and “about” may be utilizedherein to represent the inherent degree of uncertainty that may beattributed to any quantitative comparison, value, measurement, or otherrepresentation. These terms are also utilized herein to represent thedegree by which a quantitative representation may vary from a statedreference without resulting in a change in the basic function of thesubject matter at issue. Thus, a glass that is “substantially free ofMgO” is one in which MgO is not actively added or batched into theglass, but may be present in very small amounts as a contaminant; e.g.,≥0.1 mol %.

Referring to the drawings in general and to FIG. 1 in particular, itwill be understood that the illustrations are for the purpose ofdescribing particular embodiments and are not intended to limit thedisclosure or appended claims thereto. The drawings are not necessarilyto scale, and certain features and certain views of the drawings may beshown exaggerated in scale or in schematic in the interest of clarityand conciseness.

As used herein, the terms “depth of layer” and “DOL” refer to the depthof the compressive layer as determined by surface stress meter (FSM)measurements using commercially available instruments such as theFSM-6000.

As used herein, the terms “depth of compression” and “DOC” refer to thedepth at which the stress within the glass changes from compressive totensile stress. At the DOC, the stress crosses from a positive(compressive) stress to a negative (tensile) stress and thus has a valueof zero.

As described herein, compressive stress (CS) and central tension (CT)are expressed in terms of megaPascals (MPa), depth of layer (DOL) anddepth of compression (DOC) are expressed in terms of microns (μm), where1 μm=0.001 mm, and thickness t is expressed herein in terms ofmillimeters, where 1 mm=1000 unless otherwise specified.

As used herein, the term “fracture,” unless otherwise specified, meansthat a crack propagates across the entire thickness and/or entiresurface of a substrate when that substrate is dropped or impacted withan object.

According to the scientific convention normally used in the art,compression is expressed as a negative (<0) stress and tension isexpressed as a positive (>0) stress. Throughout this description,however, compressive stress CS is expressed as a positive or absolutevalue—i.e., as recited herein, CS=|CS| and central tension or tensilestress is expressed as a negative value in order to better visualize thecompressive stress profiles described herein.

As used herein, the “slope (m)” refers to the slope of a segment orportion of the stress profile that closely approximates a straight line.The predominant slope is defined as the average slope for regions thatare well approximated as straight segments. These are regions in whichthe absolute value of the second derivative of the stress profile issmaller than the ratio of the absolute value of the first derivative,and approximately half the depth of the region, as specified in equation(4) below. For a steep, shallow segment of the stress profile near thesurface of the strengthened glass article, for example, the essentiallystraight segment is the portion for each point of which the absolutevalue of the second derivative of the stress profile is smaller than theabsolute value of the local slope of the stress profile divided by thedepth at which the absolute value of the stress changes by a factor of2. Similarly, for a segment of the profile deeper within the glass, thestraight portion of the segment is the region for which the local secondderivative of the stress profile has an absolute value that is smallerthan the absolute value of the local slope of the stress profile dividedby half the DOC.

For typical stress profiles, this limit on the second derivativeguarantees that the slope changes relatively slowly with depth, and istherefore reasonably well defined and can be used to define regions ofslope that are important for the stress profiles that are consideredadvantageous for drop performance.

Let the stress as profile a function of depth x be given by the function

σ=σ(x)  (1),

and let the first derivative of the stress profile with respect to depthbe

$\begin{matrix}{{\sigma^{\prime} = \frac{d\sigma}{dx}},} & (2)\end{matrix}$

and the second derivative be

$\begin{matrix}{\sigma^{''} = {\frac{d^{2}\sigma}{dx^{2}}.}} & (3)\end{matrix}$

If a shallow segment extends approximately to a depth d_(s), then forthe purposes of defining a predominant slope, a straight portion of theprofile is a region where

$\begin{matrix}\left| {\sigma^{''}(x)} \middle| {< \left| {2\frac{\sigma^{\prime}(x)}{d_{s}}} \middle| . \right.} \right. & (4)\end{matrix}$

If a deep segment extends approximately to a larger depth DOC, or to alarger depth d_(d), or to a depth DOL in traditional terms, then astraight portion of the profile is a region where

$\begin{matrix}\left| {\sigma^{''}(x)} \middle| {< \left| {2\frac{\sigma^{\prime}(x)}{d_{d}}} \middle| {\approx \left| {2\frac{\sigma^{\prime}(x)}{DOC}} \middle| {\approx \left| {2\frac{\sigma^{\prime}(x)}{DOL}} \middle| . \right.} \right.} \right.} \right. & (5)\end{matrix}$

The latter equation is also valid for a 1-segment stress profileobtained by a single ion exchange in a salt containing only a singlealkali ion other than the ion being replaced in the glass for chemicalstrengthening.

Preferably, the straight segments are selected as regions where

$\begin{matrix}{\left| {\sigma^{''}(x)} \middle| {< \left| \frac{\sigma^{\prime}(x)}{d} \right|} \right.,} & (6)\end{matrix}$

where d stands for the relevant depth for the region, shallow or deep.

The slope m of linear segments of the compressive stress profilesdescribed herein are given as absolute values of the slope

$\frac{d\sigma}{dx}$

—i.e., m, as recited herein, is equal to

$\left| \frac{d\sigma}{dx} \right|.$

More specifically, the slope m represents the absolute value of theslope of a profile for which the compressive stress generally decreasesas a function of increasing depth.

Described herein are glass articles that are chemically strengthened byion exchange to obtain a prescribed compressive stress profile and thusachieve survivability when dropped onto a hard, abrasive surface from aprescribed height.

Compressive stress CS and depth of layer DOL are stress profileparameters that have been used for years to enable quality control ofchemical strengthening. Compressive stress CS provides an estimate ofthe surface compression, an important parameter that correlates wellwith the amount of stress that needs to be applied to cause a failure ofa glass article, particularly when the glass is free of substantiallydeep mechanical flaws. Depth of layer DOL has been used as anapproximate measure of the depth of penetration of the larger(strengthening) cation (e.g., K⁺ during K⁺ for Na⁺ exchange), withlarger DOL correlating well with greater depths of the compressionlayer, protecting the glass by arresting deeper flaws, and preventingflaws from causing failure under conditions of relatively low externallyapplied stress.

Even with minor to moderate bending of a glass article, the bendingmoment induces a stress distribution that is generally linear with depthfrom the surface, having a maximum tensile stress on the outer side ofbending, a maximum compressive stress on the inner side of the bending,and zero stress at the so-called neutral surface, which is usually inthe interior. For tempered glass parts, this bending-inducedconstant-slope stress distribution is added to the tempering stressprofile to result in the net stress profile in the presence of external(bending) stress.

The net profile in the presence of bending-induced stress generally hasa different depth of compression DOC from the stress profile withoutbending. In particular, on the outer side of bending, the depth ofcompression DOC is reduced in the presence of bending. If the temperingstress profile has a relatively small stress slope at depths in thevicinity of and smaller than the DOC, the DOC can drop verysubstantially in the presence of bending. In the net stress profile, thetips of moderately deep flaws could be exposed to tension, while thesame flaw tips would normally be arrested in the compression region ofthe tempering profile without bending. These moderately deep flaws canthus grow and lead to fracture during the bending.

Bending stresses are also important during drop testing. Regions oflocalized time-varying stress occur during mechanical vibrations andwave propagation through the glass article. With increasing drop height,the glass article experiences higher time-varying stresses duringcontact with the floor surface as well as during vibrations occurringafter contact. Thus, some fracture failures may occur due to excessivepost-contact tensile stress occurring at the tip of a relatively shallowflaw that would normally be innocuous in the presence of temperingwithout these time-varying stresses.

The present disclosure describes a range of slopes that provides a goodtrade-off between performance of the glass article during drop tests andduring bending tests. The preferable ranges may in some cases bepartially defined or limited by the capabilities and limitations ofstress measurement equipment (such as, for example, the FSM-6000 stressmeter) for collection and interpretation of spectra associated withthese profiles for the purposes of quality control during production.Not only the depth of layer DOL, but also the slope of the stressprofile (through the slope of the index profile associated with thestress profile), affect the ability to resolve particular lines in thecoupling spectra, and thus to effectively control product quality.

Ion exchange is commonly used to chemically strengthen glasses. In oneparticular example, alkali cations within a source of such cations(e.g., a molten salt, or “ion exchange,” bath) are exchanged withsmaller alkali cations within the glass to achieve a layer that is undera compressive stress (CS) near the surface of the glass. For example,potassium ions from the cation source are often exchanged with sodiumions within the glass. The compressive layer extends from the surface toa depth within the glass.

A cross-sectional schematic view of a planar ion exchanged glass articleis shown in FIG. 1. Glass article 100 has a thickness t, first surface110, and second surface 112. In some embodiments, glass article 100 hasa thickness t of at least 0.15 mm and up to about (i.e., less than orequal to) about 2.0 mm, or up to about 1.0 mm, or up to about 0.7 mm, orup to about 0.5 mm. While the embodiment shown in FIG. 1 depicts glassarticle 100 as a flat planar sheet or plate, glass article 100 may haveother configurations, such as a three dimensional shape or anothernon-planar configurations. Glass article 100 has a first compressiveregion 120 extending from first surface 110 to a depth of compression(DOC) d₁ into the bulk of the glass article 100. In the embodiment shownin FIG. 1, glass article 100 also has a second compressive region 122extending from second surface 112 to a second depth of compression (DOC)d₂. Glass article 100 also has a central region 130 that extends from d₁to d₂. Central region 130 is under a tensile stress, having a maximumvalue at the center of the central region 130, referred to as centraltension or center tension (CT). The tensile stress of region 130balances or counteracts the compressive stresses CS of regions 120 and122. The depths d₁, d₂ of first and second compressive regions 120, 122protect the glass article 100 from the propagation of flaws introducedby sharp impact to first and second surfaces 110, 112 of glass article100, while the compressive stress CS minimizes the likelihood of a flawgrowing and penetrating through the depth d₁, d₂ of first and secondcompressive regions 120, 122.

The strengthened glass articles described herein have a maximumcompressive stress CS_(s) of at least about 150 megaPascals (MPa). Insome embodiments, the maximum compressive stress CS_(s) is at leastabout 100 MPa, in other embodiments, at least 140 MPa, and, in someembodiments, up to about 400 MPa. In some embodiments, the maximumcompressive stress CS_(s) is located at the surface (110, 112 in FIG.1). In other embodiments, however, the maximum compressive CS_(s) may belocated in the compressive region (120, 122) at some depth below thesurface of the glass article. Each compressive region (120, 122) extendsfrom the surface of the glass article to a depth of compression DOC (d₁,d₂) of at least about 95 microns (μm) to about 250 μm. In someembodiments, DOC is in a range from about 100 μm and, in otherembodiment, from about 140 μm to about 190 μm. The depth of compressionDOC (d₁, d₂) may also be expressed in terms of the thickness t of theglass article 100. In some embodiments, 0.1·t≤DOC≤0.25·t, and, in otherembodiments, 0.12·t≤DOC≤0.22·t.

The compressive stress varies as a function of depth below the surfaceof the strengthened glass article, producing a compressive stressprofile in the compressive region. In some embodiments, the compressivestress profile is substantially linear with respect to depth below thesurface within the compression region, as schematically shown in FIG. 2.In FIG. 2, the compressive stress behaves substantially linearly withrespect to depth below the surface, resulting in a straight line ahaving a slope m_(a), expressed in MPa/μm, that intercepts the verticaly (CS) axis at CS_(s). CS profile an intercepts the x axis at the depthof compression DOC. At this point, the total stress(tension+compression) is zero. Below DOC, the glass article is intension CT, reaching a central value CT. In one non-limiting example,there may be a sub-region over which the tension varies from 0 up to amaximum (by absolute value) tension equal to CT, and a region over whichthe tension is substantially constant and equal to CT.

In some embodiments, the substantially linear portion of the compressivestress profile a of the glass article described herein has a slope m_(a)that is within a specified range. In FIG. 2, for example, slope m_(a) ofline a lies between upper boundary δ₂ and lower boundary δ₁; i.e.,δ₂≤m_(a)≤δ₁. In some embodiments, slope m_(a) is in a range from about−0.4 MPa/μm to about −3.0 MPa/μm. In some embodiments, −0.7MPa/μm≥m_(a)≥−2.7 MPa/μm, in other embodiments, −1.0 MPa/μm≥m_(a)≥−2.0MPa/μm and, in other embodiments, −1.5 MPa/μm≥m_(a)≥−2.7 MPa/μm. Whenthe slope m_(a) has such values and the depth of compression DOC is atleast about 95 μm, the resistance of the strengthened glass to at leastone type of failure mode (e.g., very deep puncture) that may beprevalent in field failures of certain device designs is particularlyadvantageous.

In other embodiments, the compressive stress profile is a combination ofmore than one substantially linear function, as schematically shown inFIG. 3. As seen in FIG. 3, the compressive stress profile has a firstsegment or portion b and a second segment or portion c. First portion bexhibits substantially linear behavior from the strengthened surface ofthe glass article to a depth d_(b). First portion b has a slope m_(b)and y intercept CS_(s). Second portion c of the compressive stressprofile extends from approximately depth d_(b) to the depth ofcompression DOC, and has a slope m_(c). The compressive stress CS(d_(b))at depth d_(b) is given by the expression

CS(d _(b))≈CS _(s) −d _(b)(m _(b))  (7).

In some embodiments, depth d_(b) is in a range from about 3 μm to about8 μm; i.e., 3 μm d_(b)≤8 μm. In other embodiments, 3 μm≤d_(b)≤10 μm. Instill other embodiments, 3 μm≤d_(b)≤15 μm.

It will be appreciated by those skilled in the art that the presentdisclosure is not limited to compressive stress profiles consisting ofonly two distinct portions. Instead, the compressive stress profile mayinclude additional segments. In some embodiments, different linearportions or segments of the compressive stress profile may be joined bya transitional region (not shown) in which the slope of the profiletransitions from a first slope to a second slope (e.g., from m_(b) tom_(c)).

As shown in FIG. 3, the slope of portion b of the compressive stressprofile is much steeper than the slope of portion b; i.e.,|m_(b)|>>|m_(c)|. This corresponds to a condition in which a compressivestress profile having a “spike” at the surface of the glass article iscreated by multiple ion exchange processes carried out in succession inorder to provide the surface with sufficient compressive stress towithstand the introduction or growth of some flaws produced throughimpact.

In some embodiments, the compressive stress profiles b and c of theglass article described herein have slopes m_(b) and m_(c),respectively, that are within specified ranges. In FIG. 3, for example,slope m_(b) of line/first portion b lies between upper boundary δ₃ andlower boundary δ₄ and slope m_(c) of line/first portion c lies betweenupper boundary δ₅ and lower boundary δ₆; i.e., δ₃≥m_(b)≥δ₄ andδ₅≥m_(c)≥δ₆. In some embodiments, −40 MPa/μm≥m_(b)≥−200 MPa/μm, and −0.7MPa/μm≥m_(c)≥−2.0 MPa/μm. In some embodiments, −40 MPa/μm≥m_(b)≥−120MPa/μm and, in some embodiments, −50 MPa/μm≥m_(b)≥−120 MPa/μm. In someembodiments, slope m_(c) is in a range from about −0.4 MPa/μm to about−3.0 MPa/μm. In some embodiments, −0.7 MPa/μm m_(c)≥−2.7 MPa/μm, inother embodiments, −1.0 MPa/μm≥m_(c)≥−2.0 MPa/μm and, in otherembodiments, −1.5 MPa/μm≥m_(c)≥−2.7 MPa/μm.

Compressive stress CS and depth of the compressive layer (referred to as“depth of layer” or DOL) are measured using those means known in theart. Such means include, but are not limited to, measurement of surfacestress (FSM) using commercially available instruments such as theFSM-6000, manufactured by Luceo Co., Ltd. (Tokyo, Japan), or the like.Methods of measuring compressive stress and depth of layer are describedin ASTM 1422C-99, entitled “Standard Specification for ChemicallyStrengthened Flat Glass,” and ASTM 1279.19779 “Standard Test Method forNon-Destructive Photoelastic Measurement of Edge and Surface Stresses inAnnealed, Heat-Strengthened, and Fully-Tempered Flat Glass,” thecontents of which are incorporated herein by reference in theirentirety. Surface stress measurements rely upon the accurate measurementof the stress optical coefficient (SOC), which is related to thebirefringence of the glass. The stress optical coefficient is in turnmeasured by those methods that are known in the art, such as fiber andfour point bend methods, both of which are described in ASTM standardC770-98 (2008), entitled “Standard Test Method for Measurement of GlassStress-Optical Coefficient,” the contents of which are incorporatedherein by reference in their entirety, and a bulk cylinder method.

The relationship between CS and central tension CT may, in someembodiments, be approximated by the expression:

CT=(CS·DOL)/(t−2DOL)  (8),

where t is the thickness, expressed in microns (μm), of the glassarticle. In various sections of the disclosure, central tension CT andcompressive stress CS are expressed herein in megaPascals (MPa),thickness t is expressed in either microns (μm) or millimeters (mm), anddepth of layer DOL is expressed in microns (μm) or millimeters (mm),consistent with the representation oft.

For strengthened glass articles in which the compressive stress layersextend to deeper depths within the glass, the FSM technique may sufferfrom contrast issues which affect the observed DOL value. At deeper DOLvalues, there may be inadequate contrast between the TE and TM spectra,thus making the calculation of the difference between TE and TMspectra—and thus determining the DOL—more difficult. Moreover, the FSMsoftware analysis is incapable of determining the compressive stressprofile (i.e., the variation of compressive stress as a function ofdepth within the glass). In addition, the FSM technique is incapable ofdetermining the depth of layer resulting from the ion exchange ofcertain elements such as, for example, ion exchange of sodium forlithium.

The DOL as determined by the FSM is a relatively good approximation forthe depth of compression (DOC) when the DOL is a small fraction r of thethickness t and the index profile has a depth distribution with isreasonably well approximated with a simple linear truncated profile.When the DOL is a substantial fraction of the thickness, such asDOL≥0.1·t, then the DOC is most often noticeably lower than the DOL. Forexample, in the idealized case of a linear truncated profile, therelationship DOC=DOL (1−r) holds, where r=DOL/t.

Most TM and TE index profiles have a curved portion near the bottom ofthe index profile, and the relationship between DOC and DOL then may besomewhat more involved, but generally the ratio DOC/DOL decreases as rincreases. For some profile shapes it is possible to have even DOC DOL,particularly when r<0.02.

When the concentration profile of the larger (strengthening) cation(e.g., K⁺) introduced by ion exchange has two segments, with the segmentone nearest the surface having a substantially higher concentration, andthe segment spread over large depths and having a substantially lowerconcentration, the DOL as found by the FSM is significantly smaller thanthe overall depth of chemical penetration of the larger ion. This is incontrast with the case of a simple one-segment diffusion profile inwhich the DOL provides a good estimate of the chemical penetration. In atwo-segment profile, the DOC may be larger or smaller than the DOL,depending on the depth and stress parameters of the profile and on thethickness.

When low external stresses are applied to a strengthened glass, thefracture-causing flaws have depths that correlate better with the DOCrather than the DOL. The reason why DOL has been used successfully as ahigh-value parameter of chemical strengthening is that for simplesingle-segment stress profiles, the DOL has had a good correlation withDOC. In addition, the DOC and the DOL have been similar, since for manyyears the DOL has been generally lower than 0.1·t, and for the most partlower than 0.05·t. Thus, for traditional chemically-strengthened glass,the DOL has had good correlation with the depth of strength-limitingflaws.

With the increasing importance of thinner cover glasses (e.g., havingt<0.5 mm) and the introduction of deeper and more complex stressprofiles aimed at improving drop performance while preserving highstrength under high-stress tests such as ring-on-ring (ROR), abradedring-on-ring (AROR), and four-point-bend (4PB), the depth of layer DOLdeviates significantly from the depth of compression DOC.Fracture-inducing flaws under conditions of low external stress oftenoccur at depths smaller than the DOL, but are consistent with the DOC.

The techniques described below have been developed to more accuratelydetermine the depth of compression (DOC) and compressive stress profilesfor strengthened glass articles.

In U.S. patent application Ser. No. 13/463,322, entitled “Systems AndMethods for Measuring the Stress Profile of Ion-Exchanged Glass(hereinafter referred to as “Roussev I”),” filed by Rostislav V. Roussevet al. on May 3, 2012, and claiming priority to U.S. Provisional PatentApplication No. 61/489,800, having the same title and filed on May 25,2011, two methods for extracting detailed and precise stress profiles(stress as a function of depth) of tempered or chemically strengthenedglass are disclosed. The spectra of bound optical modes for TM and TEpolarization are collected via prism coupling techniques and used intheir entirety to obtain detailed and precise TM and TE refractive indexprofiles n_(TM)(z) and n_(TE)(z). In one embodiment, the detailed indexprofiles are obtained from the mode spectra by using the inverseWentzel-Kramers-Brillouin (IWKB) method. The contents of the aboveapplications are incorporated herein by reference in their entirety.

In another embodiment, the detailed index profiles are obtained byfitting the measured mode spectra to numerically calculated spectra ofpre-defined functional forms that describe the shapes of the indexprofiles and obtaining the parameters of the functional forms from thebest fit. The detailed stress profile S(z) is calculated from thedifference of the recovered TM and TE index profiles by using a knownvalue of the stress-optic coefficient (SOC):

S(z)=[n _(TM)(z)−n _(TE)(z)]/SOC  (9).

Due to the small value of the SOC, the birefringence n_(TM)(z)−n_(TE)(z)at any depth z is a relatively small fraction (typically on the order of1%) of either of the indices n_(TM)(z) and n_(TE)(z). Obtaining stressprofiles that are not significantly distorted due to noise in themeasured mode spectra requires determination of the mode effectiveindices with precision on the order of 0.00001 RIU (refractive indexunits). The methods disclosed in Roussev I further include techniquesapplied to the raw data to ensure such high precision for the measuredmode indices, despite noise and/or poor contrast in the collected TE andTM mode spectra or images of the mode spectra. Such techniques includenoise-averaging, filtering, and curve fitting to find the positions ofthe extremes corresponding to the modes with sub-pixel resolution.

Similarly, U.S. patent application Ser. No. 14/033,954, entitled“Systems and Methods for Measuring Birefringence in Glass andGlass-Ceramics (hereinafter “Roussev II”),” filed by Rostislav V.Roussev et al. on Sep. 23, 2013, and claiming priority to U.S.Provisional Application Ser. No. 61/706,891, having the same title andfiled on Sep. 28, 2012, discloses apparatus and methods for opticallymeasuring birefringence on the surface of glass and glass ceramics,including opaque glass and glass ceramics. Unlike Roussev I, in whichdiscrete spectra of modes are identified, the methods disclosed inRoussev II rely on careful analysis of the angular intensitydistribution for TM and TE light reflected by a prism-sample interfacein a prism-coupling configuration of measurements. The contents of theabove applications are incorporated herein by reference in theirentirety.

In another disclosed method, derivatives of the TM and TE signals aredetermined after application of some combination of the aforementionedsignal conditioning techniques. The locations of the maximum derivativesof the TM and TE signals are obtained with sub-pixel resolution, and thesurface birefringence is proportional to the spacing of the above twomaxima, with a coefficient determined as before by the apparatusparameters.

Associated with the requirement for correct intensity extraction, theapparatus comprises several enhancements, such as using alight-scattering surface (static diffuser) in close proximity to or onthe prism entrance surface to improve the angular uniformity ofillumination, a moving diffuser for speckle reduction when the lightsource is coherent or partially coherent, and light-absorbing coatingson portions of the input and output facets of the prism and on the sidefacets of the prism, to reduce parasitic background which tends todistort the intensity signal. In addition, the apparatus may include aninfrared light source to enable measurement of opaque materials.

Furthermore, Roussev II discloses a range of wavelengths and attenuationcoefficients of the studied sample, where measurements are enabled bythe described methods and apparatus enhancements. The range is definedby α_(s)λ<250πσ_(s), where α_(s) is the optical attenuation coefficientat measurement wavelength λ, and σ_(s) is the expected value of thestress to be measured with typically required precision for practicalapplications. This wide range allows measurements of practicalimportance to be obtained at wavelengths where the large opticalattenuation renders previously existing measurement methodsinapplicable. For example, Roussev II discloses successful measurementsof stress-induced birefringence of opaque white glass-ceramic at awavelength of 1550 nm, where the attenuation is greater than about 30dB/mm.

While it is noted above that there are some issues with the FSMtechnique at deeper DOL values, FSM is still a beneficial conventionaltechnique which may utilized with the understanding that an error rangeof up to +/−20% is possible at deeper DOL values. The terms “depth oflayer” and “DOL” as used herein refer to DOL values computed using theFSM technique, whereas the terms “depth of compression” and “DOC” referto depths of the compressive layer determined by the methods describedin Roussev I & II.

As stated above, the glass articles may be chemically strengthened byion exchange. In this process, ions at or near the surface of the glassare replaced by—or exchanged with—larger ions usually having the samevalence or oxidation state. In those embodiments in which the glassarticle comprises, consists essentially of, or consists of an alkalialuminosilicate glass, ions in the surface layer of the glass and thelarger ions are monovalent alkali metal cations, such as Na⁺ (when Li⁺is present in the glass), K⁺, Rb⁺, and Cs⁺. Alternatively, monovalentcations in the surface layer may be replaced with monovalent cationsother than alkali metal cations, such as Ag⁺ or the like.

Ion exchange processes are typically carried out by immersing a glassarticle in a molten salt bath containing the larger ions to be exchangedwith the smaller ions in the glass. It will be appreciated by thoseskilled in the art that parameters for the ion exchange process,including, but not limited to, bath composition and temperature,immersion time, the number of immersions of the glass in a salt bath (orbaths), use of multiple salt baths, additional steps such as annealing,washing, and the like, are generally determined by the composition ofthe glass and the desired depth of layer and compressive stress of theglass that result from the strengthening operation. By way of example,ion exchange of alkali metal-containing glasses may be achieved byimmersion in at least one molten bath containing a salt such as, but notlimited to, nitrates, sulfates, and chlorides of the larger alkali metalion. The temperature of the molten salt bath typically is in a rangefrom about 380° C. up to about 450° C., while immersion times range fromabout 15 minutes up to about 40 hours. However, temperatures andimmersion times different from those described above may also be used.

In addition, non-limiting examples of ion exchange processes in whichglass is immersed in multiple ion exchange baths, with washing and/orannealing steps between immersions, are described in U.S. Pat. No.8,561,429, by Douglas C. Allan et al., issued on Oct. 22, 2013, entitled“Glass with Compressive Surface for Consumer Applications,” and claimingpriority from U.S. Provisional Patent Application No. 61/079,995, filedJul. 11, 2008, in which glass is strengthened by immersion in multiple,successive, ion exchange treatments in salt baths of differentconcentrations; and U.S. Pat. No. 8,312,739, by Christopher M. Lee etal., issued on Nov. 20, 2012, and entitled “Dual Stage Ion Exchange forChemical Strengthening of Glass,” and claiming priority from U.S.Provisional Patent Application No. 61/084,398, filed Jul. 29, 2008, inwhich glass is strengthened by ion exchange in a first bath is dilutedwith an effluent ion, followed by immersion in a second bath having asmaller concentration of the effluent ion than the first bath. Thecontents of U.S. Pat. Nos. 8,561,429 and 8,312,739 are incorporatedherein by reference in their entirety.

The compressive stress is created by chemically strengthening the glassarticle, for example, by the ion exchange processes previously describedherein, in which a plurality of first metal ions in the outer region ofthe glass article is exchanged with a plurality of second metal ions sothat the outer region comprises the plurality of the second metal ions.Each of the first metal ions has a first ionic radius and each of thesecond alkali metal ions has a second ionic radius. The second ionicradius is greater than the first ionic radius, and the presence of thelarger second alkali metal ions in the outer region creates thecompressive stress in the outer region.

At least one of the first metal ions and second metal ions are ions ofan alkali metal. The first ions may be ions of lithium, sodium,potassium, and rubidium. The second metal ions may be ions of one ofsodium, potassium, rubidium, and cesium, with the proviso that thesecond alkali metal ion has an ionic radius greater than the ionicradius than the first alkali metal ion.

In some embodiments, the glass is strengthened in a single ion exchangestep to produce the compressive stress profile shown in FIG. 2.Typically, the glass is immersed in a molten salt bath containing a saltof the larger alkali metal cation. In some embodiments, the molten saltbath contains or consists essentially of salts of the larger alkalimetal cation. However, small amounts—in some embodiments, less thatabout 10 wt %, in some embodiments, less than about 5 wt %, and, inother embodiments less than about 2 wt %—of salts of the smaller alkalimetal cation may be present in the bath. In other embodiments, salts ofthe smaller alkali metal cation may comprise at least about 30 wt %, orat least about 40 wt %, or from about 40 wt % to about 75 wt % of theion exchange bath. This single ion exchange process may take place at atemperature of at least about 400° C. and, in some embodiments, at leastabout 440° C., for a time sufficient to achieve the desired depth ofcompression DOC. In some embodiments, the single step ion exchangeprocess may be conducted for at least eight hours, depending on thecomposition of the bath.

In another embodiment, the glass is strengthened in a two-step or dualion exchange method to produce the compressive stress profile shown inFIG. 3. The first step of the process, the glass is ion exchanged in thefirst molten salt bath described above. After completion of the firstion exchange, the glass is immersed in a second ion exchange bath. Thesecond ion exchange bath is different—i.e., separate from and, in someembodiments, having a different composition—from the first bath. In someembodiments, the second ion exchange bath contains only salts of thelarger alkali metal cation, although, in some embodiments small amountsof the smaller alkali metal cation (e.g., ≤2 wt %; ≤3 wt %) may bepresent in the bath. in addition, the immersion time and temperature ofthe second ion exchange step may differ from those of the first ionexchange step. In some embodiments, the second ion exchange step iscarried out at a temperature of at least about 350° C. and, in otherembodiments, at least about 380° C. The duration of the second ionexchange step is sufficient to achieve the desired depth d_(a) of theshallow segment, in some embodiments, may be 30 minutes or less. Inother embodiments, the duration is 15 minutes or less and, in someembodiments, in a range from about 10 minutes to about 60 minutes.

The second ion exchange bath is different than the first ion exchangebath, because the second ion exchange step is directed to delivering adifferent concentration of the larger cation or, in some embodiments, adifferent cation altogether, to the alkali aluminosilicate glass articlethan the first ion exchange step. In one or more embodiments, the secondion exchange bath may comprise at least about 95% by weight of apotassium composition that delivers potassium ions to the alkalialuminosilicate glass article. In a specific embodiment, the second ionexchange bath may comprise from about 98% to about 99.5% by weight ofthe potassium composition. While it is possible that the second ionexchange bath only comprises at least one potassium salt, the second ionexchange bath may, in further embodiments, comprise 0-5% by weight, orabout 0.5-2.5% by weight of at least one sodium salt, for example,NaNO₃. In an exemplary embodiment, the potassium salt is KNO₃. Infurther embodiments, the temperature of the second ion exchange step maybe 380° C. or greater.

The purpose of the second ion exchange step is to form a “spike”increase the compressive stress in the region immediately adjacent tothe surface of the glass article, as represented by portion b of thestress profile shown in FIG. 3.

The glass articles described herein may comprise or consist essentiallyof any glass that is chemically strengthened by ion exchange. In someembodiments, the glass is an alkali aluminosilicate glass.

In one embodiment, the alkali aluminosilicate glass comprises orconsists essentially of at least one of alumina and boron oxide, and atleast one of an alkali metal oxide and an alkali earth metal oxide,wherein −15 mol % (R₂O+R′O−Al₂O₃−ZrO₂)−B₂O₃≤4 mol %, where R is one ofLi, Na, K, Rb, and Cs, and R′ is at least one of Mg, Ca, Sr, and Ba. Insome embodiments, the alkali aluminosilicate glass comprises or consistsessentially of: from about 62 mol % to about 70 mol. % SiO₂; from 0 mol% to about 18 mol % Al₂O₃; from 0 mol % to about 10 mol % B₂O₃; from 0mol % to about 15 mol % Li₂O; from 0 mol % to about 20 mol % Na₂O; from0 mol % to about 18 mol % K₂O; from 0 mol % to about 17 mol % MgO; from0 mol % to about 18 mol % CaO; and from 0 mol % to about 5 mol % ZrO₂.In some embodiments, the glass comprises alumina and boron oxide and atleast one alkali metal oxide, wherein −15 mol%≤(R₂O+R′O−Al₂O₃−ZrO₂)−B₂O₃≤4 mol %, where R is at least one of Li, Na,K, Rb, and Cs, and R′ is at least one of Mg, Ca, Sr, and Ba; wherein10≤Al₂O₃+B₂O₃+ZrO₂≤30 and 14≤R₂O+R′O≤25; wherein the silicate glasscomprises or consists essentially of: 62-70 mol. % SiO₂; 0-18 mol %Al₂O₃; 0-10 mol % B₂O₃; 0-15 mol % Li₂O; 6-14 mol % Na₂O; 0-18 mol %K₂O; 0-17 mol % MgO; 0-18 mol % CaO; and 0-5 mol % ZrO₂. The glass isdescribed in U.S. patent application Ser. No. 12/277,573 filed Nov. 25,2008, by Matthew J. Dejneka et al., and entitled “Glasses HavingImproved Toughness And Scratch Resistance,” and U.S. Pat. No. 8,652,978filed Aug. 17, 2012, by Matthew J. Dejneka et al., and entitled “GlassesHaving Improved Toughness And Scratch Resistance,” both claimingpriority to U.S. Provisional Patent Application No. 61/004,677, filed onNov. 29, 2008. The contents of all of the above are incorporated hereinby reference in their entirety.

In another embodiment, the alkali aluminosilicate glass comprises orconsists essentially of: from about 60 mol % to about 70 mol % SiO₂;from about 6 mol % to about 14 mol % Al₂O₃; from 0 mol % to about 15 mol% B₂O₃; from 0 mol % to about 15 mol % Li₂O; from 0 mol % to about 20mol % Na₂O; from 0 mol % to about 10 mol % K₂O; from 0 mol % to about 8mol % MgO; from 0 mol % to about 10 mol % CaO; from 0 mol % to about 5mol % ZrO₂; from 0 mol % to about 1 mol % SnO₂; from 0 mol % to about 1mol % CeO₂; less than about 50 ppm As₂O₃; and less than about 50 ppmSb₂O₃; wherein 12 mol %≤Li₂O+Na₂O+K₂O≤20 mol % and 0 mol %≤MgO+CaO≤10mol %. In some embodiments, the alkali aluminosilicate glass comprisesor consists essentially of: 60-70 mol % SiO₂; 6-14 mol % Al₂O₃; 0-3 mol% B₂O₃; 0-1 mol % Li₂O; 8-18 mol % Na₂O; 0-5 mol % K₂O; 0-2.5 mol % CaO;greater than 0 mol % to 3 mol % ZrO₂; 0-1 mol % SnO₂; and 0-1 mol %CeO₂, wherein 12 mol %<Li₂O+Na₂O+K₂O≤20 mol %, and wherein the silicateglass comprises less than 50 ppm As₂O₃. In some embodiments, the alkalialuminosilicate glass comprises or consists essentially of: 60-72 mol %SiO₂; 6-14 mol % Al₂O₃; 0-3 mol % B₂O₃; 0-1 mol % Li₂O; 0-20 mol % Na₂O;0-10 mol % K₂O; 0-2.5 mol % CaO; 0-5 mol % ZrO₂; 0-1 mol % SnO₂; and 0-1mol % CeO₂, wherein 12 mol % Li₂O+Na₂O+K₂O≤20 mol %, and wherein thesilicate glass comprises less than 50 ppm As₂O₃ and less than 50 ppmSb₂O₃. The glass is described in U.S. Pat. No. 8,158,543 by Sinue Gomezet al., entitled “Fining Agents for Silicate Glasses,” filed on Feb. 25,2009; U.S. Pat. No. 8,431,502 by Sinue Gomez et al., entitled “SilicateGlasses Having Low Seed Concentration,” filed Jun. 13, 2012; and U.S.Pat. No. 8,623,776, by Sinue Gomez et al., entitled “Silicate GlassesHaving Low Seed Concentration,” filed Jun. 19, 2013, all of which claimpriority to U.S. Provisional Patent Application No. 61/067,130, filed onFeb. 26, 2008. The contents of all of the above are incorporated hereinby reference in their entirety.

In another embodiment, the alkali aluminosilicate glass comprises SiO₂and Na₂O, wherein the glass has a temperature T_(35kp) at which theglass has a viscosity of 35 kilo poise (kpoise), wherein the temperatureT_(breakdown) at which zircon breaks down to form ZrO₂ and SiO₂ isgreater than T_(35kp). In some embodiments, the alkali aluminosilicateglass comprises or consists essentially of: from about 61 mol % to about75 mol % SiO₂; from about 7 mol % to about 15 mol % Al₂O₃; from 0 mol %to about 12 mol % B₂O₃; from about 9 mol % to about 21 mol % Na₂O; from0 mol % to about 4 mol % K₂O; from 0 mol % to about 7 mol % MgO; andfrom 0 mol % to about 3 mol % CaO. The glass is described in U.S. Pat.No. 8,802,581 by Matthew J. Dejneka et al., entitled “Zircon CompatibleGlasses for Down Draw,” filed Aug. 10, 2010, and claiming priority toU.S. Provisional Patent Application No. 61/235,762, filed on Aug. 29,2009. The contents of the above patent and application are incorporatedherein by reference in their entirety.

In another embodiment, the alkali aluminosilicate glass comprises atleast 50 mol % SiO₂ and at least one modifier selected from the groupconsisting of alkali metal oxides and alkaline earth metal oxides,wherein [(Al₂O₃ (mol %)+B₂O₃ (mol %))/(Σ alkali metal modifiers (mol%))]>1. In some embodiments, the alkali aluminosilicate glass comprisesor consists essentially of: from 50 mol % to about 72 mol % SiO₂; fromabout 9 mol % to about 17 mol % Al₂O₃; from about 2 mol % to about 12mol % B₂O₃; from about 8 mol % to about 16 mol % Na₂O; and from 0 mol %to about 4 mol % K₂O. In some embodiments, the glass comprises orconsists essentially of: at least 58 mol % SiO₂; at least 8 mol % Na₂O;from 5.5 mol % to 12 mol % B₂O₃; and Al₂O₃, wherein [(Al₂O₃ (mol %)+B₂O₃(mol %))/(Σ alkali metal modifiers (mol %))]>1, Al₂O₃ (mol %)>B₂O₃ (mol%), 0.9<R₂O/Al₂O₃<1.3. The glass is described in U.S. Pat. No.8,586,492, entitled “Crack And Scratch Resistant Glass and EnclosuresMade Therefrom,” filed Aug. 18, 2010, by Kristen L. Barefoot et al., andU.S. patent application Ser. No. 14/082,847, entitled “Crack And ScratchResistant Glass and Enclosures Made Therefrom,” filed Nov. 18, 2013, byKristen L. Barefoot et al., both claiming priority to U.S. ProvisionalPatent Application No. 61/235,767, filed on Aug. 21, 2009. The contentsof all of the above are incorporated herein by reference in theirentirety.

In another embodiment, the alkali aluminosilicate glass comprises SiO₂,Al₂O₃, P₂O₅, and at least one alkali metal oxide (R₂O), wherein0.75≤[(P₂O₅ (mol %)+R₂O (mol %))/M₂O₃ (mol %)]≤1.2, whereM₂O₃=Al₂O₃+B₂O₃. In some embodiments, the alkali aluminosilicate glasscomprises or consists essentially of: from about 40 mol % to about 70mol % SiO₂; from 0 mol % to about 28 mol % B₂O₃; from 0 mol % to about28 mol % Al₂O₃; from about 1 mol % to about 14 mol % P₂O₅; and fromabout 12 mol % to about 16 mol % R₂O and, in certain embodiments, fromabout 40 to about 64 mol % SiO₂; from 0 mol % to about 8 mol % B₂O₃;from about 16 mol % to about 28 mol % Al₂O₃; from about 2 mol % to about12 mol % P₂O₅; and from about 12 mol % to about 16 mol % R₂O. The glassis described in U.S. patent application Ser. No. 13/305,271 by Dana C.Bookbinder et al., entitled “Ion Exchangeable Glass with DeepCompressive Layer and High Damage Threshold,” filed Nov. 28, 2011, andclaiming priority to U.S. Provisional Patent Application No. 61/417,941,filed Nov. 30, 2010. The contents of the above applications areincorporated herein by reference in their entirety.

In still another embodiment, the alkali aluminosilicate glass comprisesat least about 50 mol % SiO₂ and at least about 11 mol % Na₂O, and has asurface compressive stress of at least about 900 MPa. In someembodiments, the glass further comprises Al₂O₃ and at least one of B₂O₃,K₂O, MgO and ZnO, wherein−340+27.1.Al₂O₃−28.7.B₂O₃+15.6.Na₂O−61.4.K₂O+8.1.(MgO+ZnO)≥0 mol %. Inparticular embodiments, the glass comprises or consists essentially of:from about 7 mol % to about 26 mol % Al₂O₃; from 0 mol % to about 9 mol% B₂O₃; from about 11 mol % to about 25 mol % Na₂O; from 0 mol % toabout 2.5 mol % K₂O; from 0 mol % to about 8.5 mol % MgO; and from 0 mol% to about 1.5 mol % CaO. The glass is described in U.S. patentapplication Ser. No. 13/533,298, by Matthew J. Dejneka et al., entitled“Ion Exchangeable Glass with High Compressive Stress,” filed Jun. 26,2012, and claiming priority to U.S. Provisional Patent Application No.61/503,734, filed Jul. 1, 2011. The contents of the above applicationsare incorporated herein by reference in their entirety.

In other embodiments, the alkali aluminosilicate glass is ionexchangeable and comprises: at least about 50 mol % SiO₂; at least about10 mol % R₂O, wherein R₂O comprises Na₂O; Al₂O₃; and B₂O₃, whereinB₂O₃−(R₂O−Al₂O₃)≥3 mol %. In some embodiments, the glass comprises: atleast about 50 mol % SiO₂; at least about 10 mol % R₂O, wherein R₂Ocomprises Na₂O; Al₂O₃, wherein Al₂O₃ (mol %)<R₂O (mol %); and from 3mol5 to 4.5 mol % B₂O₃, wherein B₂O₃ (mol %)−(R₂O (mol %)−Al₂O₃ (mol%))≤3 mol %. In certain embodiments, the glass comprises or consistsessentially of: at least about 50 mol % SiO₂; from about 9 mol % toabout 22 mol % Al₂O₃; from about 3 mol % to about 10 mol % B₂O₃; fromabout 9 mol % to about 20 mol % Na₂O; from 0 mol % to about 5 mol % K₂O;at least about 0.1 mol % MgO, ZnO, or combinations thereof, wherein0≤MgO≤6 and 0≤ZnO≤6 mol %; and, optionally, at least one of CaO, BaO,and SrO, wherein 0 mol %≤CaO+SrO+BaO≤2 mol %. When ion exchanged, theglass, in some embodiments, has a Vickers crack initiation threshold ofat least about 10 kgf. Such glasses are described in U.S. patentapplication Ser. No. 14/197,658, filed May 28, 2013, by Matthew J.Dejneka et al., entitled “Zircon Compatible, Ion Exchangeable Glass withHigh Damage Resistance,” which is a continuation of U.S. patentapplication Ser. No. 13/903,433, filed May 28, 2013, by Matthew J.Dejneka et al., entitled “Zircon Compatible, Ion Exchangeable Glass withHigh Damage Resistance,” both claiming priority to Provisional PatentApplication No. 61/653,489, filed May 31, 2012. The contents of theseapplications are incorporated herein by reference in their entirety.

In some embodiments, the glass comprises: at least about 50 mol % SiO₂;at least about 10 mol % R₂O, wherein R₂O comprises Na₂O; Al₂O₃, wherein−0.5 mol % Al₂O₃ (mol %)−R₂O (mol %)≤2 mol %; and B₂O₃, and wherein B₂O₃(mol %)−(R₂O (mol %)−Al₂O₃ (mol %))≥4.5 mol %. In other embodiments, theglass has a zircon breakdown temperature that is equal to thetemperature at which the glass has a viscosity of greater than about 40kPoise and comprises: at least about 50 mol % SiO₂; at least about 10mol % R₂O, wherein R₂O comprises Na₂O; Al₂O₃; and B₂O₃, wherein B₂O₃(mol %)−(R₂O (mol %)−Al₂O₃ (mol %)) 4.5 mol %. In still otherembodiments, the glass is ion exchanged, has a Vickers crack initiationthreshold of at least about 30 kgf, and comprises: at least about 50 mol% SiO₂; at least about 10 mol % R₂O, wherein R₂O comprises Na₂O; Al₂O₃,wherein −0.5 mol % Al₂O₃ (mol %)−R₂O (mol %)≤2 mol %; and B₂O₃, whereinB₂O₃ (mol %)−(R₂O (mol %)−Al₂O₃ (mol %)) 4.5 mol %. Such glasses aredescribed in U.S. patent application Ser. No. 13/903,398, by Matthew J.Dejneka et al., entitled “Ion Exchangeable Glass with High DamageResistance,” filed May 28, 2013, claiming priority from U.S. ProvisionalPatent Application No. 61/653,485, filed May 31, 2012. The contents ofthese applications are incorporated herein by reference in theirentirety.

In certain embodiments, the alkali aluminosilicate glass comprises atleast about 4 mol % P₂O₅, wherein (M₂O₃ (mol %)/R_(x)O (mol %))<1,wherein M₂O₃=Al₂O₃+B₂O₃, and wherein R_(x)O is the sum of monovalent anddivalent cation oxides present in the alkali aluminosilicate glass. Insome embodiments, the monovalent and divalent cation oxides are selectedfrom the group consisting of Li₂O, Na₂O, K₂O, Rb₂O, Cs₂O, MgO, CaO, SrO,BaO, and ZnO. In some embodiments, the glass comprises 0 mol % B₂O₃. Insome embodiments, the glass is ion exchanged to a depth of layer of atleast about 10 μm and comprises at least about 4 mol % P₂O₅, wherein0.6<[M₂O₃ (mol %)/R_(x)O (mol %)]<1.4 or 1.3<[(P₂O₅+R₂O)/M₂O₃]≤2.3,where M₂O₃=Al₂O₃+B₂O₃, R_(x)O is the sum of monovalent and divalentcation oxides present in the alkali aluminosilicate glass, and R₂O isthe sum of monovalent cation oxides present in the alkalialuminosilicate glass. In one embodiment, the glass comprises at leastabout 4 mol % P₂O₅ and from 0 mol % to about 4 mol % B₂O₃, wherein1.3<[(P₂O₅+R₂O)/M₂O₃]≤2.3, where M₂O₃=Al₂O₃+B₂O₃, and R₂O is the sum ofmonovalent cation oxides present in the alkali aluminosilicate glass. Insome embodiments, the glass is lithium-free and consists essentially offrom about 40 mol % to about 70 mol % SiO₂; from about 11 mol % to about25 mol % Al₂O₃; from about 4 mol % to about 15 mol % P₂O₅; from about 13mol % to about 25 mol % Na₂O; from about 13 to about 30 mol % R_(x)O,where wherein R_(x)O is the sum of the alkali metal oxides, alkalineearth metal oxides, and transition metal monoxides present in the glass;from about 11 to about 30 mol % M₂O₃, where M₂O₃=Al₂O₃+B₂O₃; from 0 mol% to about 1 mol % K₂O; from 0 mol % to about 4 mol % B₂O₃, and 3 mol %or less of one or more of TiO₂, MnO, Nb₂O₅, MoO₃, Ta₂O₅, WO₃, ZrO₂,Y₂O₃, La₂O₃, HfO₂, CdO, SnO₂, Fe₂O₃, CeO₂, As₂O₃, Sb₂O₃, Cl, and Br; theglass is lithium-free; and 1.3<[(P₂O₅+R₂O)/M₂O₃]≤2.3, where R₂O is thesum of monovalent cation oxides present in the glass. The glass isdescribed in U.S. patent application Ser. No. 13/678,013 by Timothy M.Gross, entitled “Ion Exchangeable Glass with High Crack InitiationThreshold,” filed Nov. 15, 2012, and U.S. Pat. No. 8,756,262 by TimothyM. Gross, entitled “Ion Exchangeable Glass with High Crack InitiationThreshold,” filed Nov. 15, 2012, both claiming priority to U.S.Provisional Patent Application No. 61/560,434 filed Nov. 16, 2011. Thecontents of the above patent and patent application are incorporatedherein by reference in their entirety.

In other embodiments, the alkali aluminosilicate glass comprises: fromabout 50 mol % to about 72 mol % SiO₂; from about 12 mol % to about 22mol % Al₂O₃; up to about 15 mol % B₂O₃; up to about 1 mol % P₂O₅; fromabout 11 mol % to about 21 mol % Na₂O; up to about 5 mol % K₂O; up toabout 4 mol % MgO; up to about 5 mol % ZnO; and up to about 2 mol % CaO.In some embodiments, the glass comprises: from about 55 mol % to about62 mol % SiO₂; from about 16 mol % to about 20 mol % Al₂O₃; from about 4mol % to about 10 mol % B₂O₃; from about 14 mol % to about 18 mol %Na₂O; from about 0.2 mol % to about 4 mol % K₂O; up to about 0.5 mol %MgO; up to about 0.5 mol % ZnO; and up to about 0.5 mol % CaO, whereinthe glass is substantially free of P₂O₅. In some embodiments,Na₂O+K₂O−Al₂O₃2.0 mol % and, in certain embodiments Na₂O+K₂O−Al₂O₃≤0.5mol %. In some embodiments, B₂O₃−(Na₂O+K₂O−Al₂O₃)>4 mol % and, incertain embodiments, B₂O₃−(Na₂O+K₂O−Al₂O₃)>1 mol %. In some embodiments,24 mol %≤RAlO₄≤45 mol %, and, in other embodiments, 28 mol %<RAlO₄<45mol %, where R is at least one of Na, K, and Ag. The glass is describedin U.S. Provisional Patent Application No. 61/909,049 by Matthew J.Dejneka et al., entitled “Fast Ion Exchangeable Glasses with HighIndentation Threshold,” filed Nov. 26, 2013, the contents of which areincorporated herein by reference in their entirety.

In some embodiments, the glasses described herein are substantially freeof at least one of arsenic, antimony, barium, strontium, bismuth, andtheir compounds. In other embodiments, the glasses may include up toabout 0.5 mol % Li₂O, or up to about 5 mol % Li₂O or, in someembodiments, up to about 10 mol % Li₂O. In other embodiments, theseglasses are free of Li₂O.

In some embodiments, the glasses described herein, when ion exchanged,are resistant to introduction of flaws by sharp or sudden impact.Accordingly, these ion exchanged glasses exhibit Vickers crackinitiation threshold of at least about 10 kilogram force (kgf) up toabout 50 kgf. In certain embodiments, these glasses exhibit a Vickerscrack initiation threshold of at least 20 kgf and, in some embodiments,at least about 30 kgf.

The glasses described herein may, in some embodiments, be down-drawableby processes known in the art, such as slot-drawing, fusion drawing,re-drawing, and the like, and have a liquidus viscosity of at least 130kilopoise. In addition to those compositions listed hereinabove, variousother ion exchangeable alkali aluminosilicate glass compositions may beused.

The strengthened glasses described herein are considered suitable forvarious two- and three-dimensional shapes and may be utilized in variousapplications, and various thicknesses are contemplated herein. In someembodiments, the glass article has a thickness in a range from about 0.1mm up to about 1.5 mm. In some embodiments, the glass article has athickness in a range from about 0.1 mm up to about 1.0 mm and, incertain embodiments, from about 0.1 mm up to about 0.5 mm.

Strengthened glass articles may also be defined by their central tensionCT. In one or more embodiments, the strengthened glass articlesdescribed herein have a CT≤150 MPa, or a CT≤125 MPa, or CT≤100 MPa. Thecentral tension of the strengthened glass correlates to the frangiblebehavior of the strengthened glass article.

In another aspect, a method of making a strengthened glass articlehaving at least one compressive stress layer extending from a surface ofthe strengthened glass article to a depth of compression DOC of at leastabout 125 μm is provided. The method includes, in some embodiments, asingle ion exchange step in which an alkali aluminosilicate glassarticle is immersed in a first ion exchange bath at a temperature ofgreater than 400° C. for a time sufficient such that the compressivestress layer has a depth of compression of at least about 100 MPa and,in other embodiments, at least about 140 MPa and up to about 400 MPaafter the ion exchange step.

Actual immersion times in the ion exchange bath may depend upon factorssuch as the temperature and/or composition of the ion exchange bath, thediffusivity of the cations within the glass, and the like. Accordingly,various time periods for ion exchange are contemplated as beingsuitable. In those instances in which potassium cations from the ionexchange bath are exchanged for sodium cations in the glass, the bathtypically comprises potassium nitrate (KNO₃). Here, the ion exchangestep, in some embodiments, may be conducted for a time of at least 5hours. Longer ion exchange periods for the ion exchange step maycorrelate with larger sodium ion contents in the first ion exchangebath. in some embodiments, the desired sodium ion content in first ionexchange bath may be achieved, for example, by including at least about30% by weight or, in some embodiments, at least about 40% by weight of asodium compound such as sodium nitrate (NaNO₃) or the like in the firstion exchange bath. In some embodiments, the sodium compound accounts forabout 40% to about 60% by weight of the first ion exchange bath. In anexemplary embodiment, the first ion exchange step is carried out at atemperature of about 440° C. or greater and, in some embodiments, up toabout 500° C.

After the first ion exchange step is performed, the strengthened glassarticle may have a maximum compressive stress (CS) of at least about 100MPa and, in other embodiments, at least 140 MPa and, in someembodiments, up to about 400 MPa. The first ion exchange step achieves acompressive layer depth/depth of compression DOC of about 100 μm toabout 200 μm and, in some embodiments, about 140 μm to 200 μm after thefirst ion exchange step.

In some embodiments, a second ion exchange step may be conducted byimmersing the alkali aluminosilicate glass article in a second ionexchange bath at a temperature of at least 350° C. up to about 450° C.for a time sufficient to produce the shallow steep segment with a depthd_(b) (FIG. 3) of at least about 3 μm following the ion exchange stepdescribed hereinabove. In some embodiments, the second ion exchange bathdiffers in composition and/or temperature from the first ion exchangebath. The second ion exchange step achieves a compressive stress at thesurface of at least about 400 MPa to about 1200 MPa.

The second ion exchange step is a relatively rapid ion exchange stepthat yields a “spike” of compressive stress near the surface of theglass as depicted in FIG. 3. In one or more embodiments, the second ionexchange step may be conducted for a time of up to about 30 minutes or,in other embodiments, up to about 15 minutes or, in some embodiments, ina range from about 10 minutes to about 60 minutes.

The second ion exchange step is directed to delivering a different ionto the alkali aluminosilicate glass article than the ion provided by thefirst ion exchange step. The composition of the second ion exchange baththerefore differs from the first ion exchange bath. In some embodiments,the second ion exchange bath comprises at least about 95% by weight of apotassium composition (e.g., KNO₃) that delivers potassium ions to thealkali aluminosilicate glass article. In a specific embodiment, thesecond ion exchange bath may comprise from about 98% to about 99.5% byweight of the potassium composition. While it is possible that thesecond ion exchange bath comprises only a potassium salt (or salts), thesecond ion exchange bath may, in further embodiments, comprise up toabout 2% by weight or from about 0.5% to about 1.5% by weight of asodium composition such as, for example, NaNO₃. In further embodiments,the temperature of the second ion exchange step may be 390° C. orgreater.

Frangible behavior is characterized by at least one of: breaking of thestrengthened glass article (e.g., a plate or sheet) into multiple smallpieces (e.g., 1 mm); the number of fragments formed per unit area of theglass article; multiple crack branching from an initial crack in theglass article; violent ejection of at least one fragment to a specifieddistance (e.g., about 5 cm, or about 2 inches) from its originallocation; and combinations of any of the foregoing breaking (size anddensity), cracking, and ejecting behaviors. As used herein, the terms“frangible behavior” and “frangibility” refer to those modes of violentor energetic fragmentation of a strengthened glass article absent anyexternal restraints, such as coatings, adhesive layers, or the like.While coatings, adhesive layers, and the like may be used in conjunctionwith the strengthened glass articles described herein, such externalrestraints are not used in determining the frangibility or frangiblebehavior of the glass articles.

Examples of frangible behavior and non-frangible behavior ofstrengthened glass articles upon point impact with a scribe having asharp tungsten carbide (WC) tip are shown in FIGS. 4a and 4b . The pointimpact test that is used to determine frangible behavior includes anapparatus that is delivered to the surface of the glass article with aforce that is just sufficient to release the internally stored energypresent within the strengthened glass article. That is, the point impactforce is sufficient to create at least one new crack at the surface ofthe strengthened glass sheet and extend the crack through thecompressive stress CS region (i.e., depth of layer) into the region thatis under central tension CT. The impact energy needed to create oractivate the crack in a strengthened glass sheet depends upon thecompressive stress CS and depth of layer DOL of the article, and thusupon the conditions under which the sheet was strengthened (i.e., theconditions used to strengthen a glass by ion exchange). Otherwise, eachion exchanged glass plate shown in FIGS. 13a and 13b was subjected to asharp dart indenter (e.g., a scribe with a sharp WC point) contactsufficient to propagate a crack into the inner region of the plate, theinner region being under tensile stress. The force applied to the glassplate was just sufficient to reach the beginning of the inner region,thus allowing the energy that drives the crack to come from the tensilestresses in the inner region rather than from the force of the dartimpact on the outer surface. The degree of ejection may be determined,for example, by centering the glass sample on a grid, impacting thesample and measuring the ejection distance of individual pieces usingthe grid.

Referring to FIG. 4a , glass plate a can be classified as beingfrangible. In particular, glass plate a fragmented into multiple smallpieces that were ejected, and exhibited a large degree of crackbranching from the initial crack to produce the small pieces.Approximately 50% of the fragments are less than 1 mm in size, and it isestimated that about 8 to 10 cracks branched from the initial crack.Glass pieces were also ejected about 5 cm from original glass plate a,as seen in FIG. 4a . A glass article that exhibits any of the threecriteria (i.e., multiple crack branching, ejection, and extremefragmentation) described hereinabove is classified as being frangible.For example, if a glass exhibits excessive branching alone but does notexhibit ejection or extreme fragmentation as described above, the glassis still characterized as frangible.

Glass plates b, c, (FIG. 4b ) and d (FIG. 4a ) are classified as notfrangible. In each of these samples, the glass sheet has broken into asmall number of large pieces. Glass plate b (FIG. 4a ), for example, hasbroken into two large pieces with no crack branching; glass plate c(FIG. 4b ) has broken into four pieces with two cracks branching fromthe initial crack; and glass plate d (FIG. 14a ) has broken into fourpieces with two cracks branching from the initial crack. Based on theabsence of ejected fragments (i.e., no glass pieces forcefully ejectedmore than 2 inches from their original location), no visible fragmentsthat are less than or equal to 1 mm in size, and the minimal amount ofobserved crack branching, samples b, c, and d are classified asnon-frangible or substantially non-frangible.

Based on the foregoing, a frangibility index (Table 1) can beconstructed to quantify the degree of frangible or non-frangiblebehavior of a glass, glass ceramic, and/or a ceramic article upon impactwith another object. Index numbers, ranging from 1 for non-frangiblebehavior to 5 for highly frangible behavior, have been assigned todescribe different levels of frangibility or non-frangibility. Using theindex, frangibility can be characterized in terms of numerousparameters: 1) the percentage of the population of fragments having adiameter (i.e., maximum dimension) of less than 1 mm (“Fragment size” inTable 1); 2) the number of fragments formed per unit area (in thisinstance, cm²) of the sample (“Fragment density” in Table 1); 3) thenumber of cracks branching from the initial crack formed upon impact(“Crack branching” in Table 1); and 4) the percentage of the populationof fragments that is ejected upon impact more than about 5 cm (or about2 inches) from their original position (“Ejection” in Table 1).

TABLE 1 Criteria for determining the degree of frangibility andfrangibility index. Fragment Fragment size density Ejection Degree ofFrangibility (% ≤ 1 (fragments/ Crack (% ≥ 5 frangibility index mm) cm²)branching cm) High 5 >20 >7 >9 >6 Medium 4 10 < n ≤ 20 5 < n ≤ 7 7 < n ≤9 4 < n ≤ 6 Low 3  5 < n ≤ 10 3 < n ≤ 5 5 < n ≤ 7 2 < n ≤ 4 None 2 0 < n≤ 5 l < n ≤ 3 2 < n ≤ 5 0 < n ≤ 2 1  0 n ≤ 1 n ≤ 2  0

A frangibility index is assigned to a glass article if the article meetsat least one of the criteria associated with a particular index value.Alternatively, if a glass article meets criteria between two particularlevels of frangibility, the article may be assigned a frangibility indexrange (e.g., a frangibility index of 2-3). The glass article may beassigned the highest value of frangibility index, as determined from theindividual criteria listed in Table 1. In many instances, it is notpossible to ascertain the values of each of the criteria, such as thefragmentation density or percentage of fragments ejected more than 5 cmfrom their original position, listed in Table 1. The different criteriaare thus considered individual, alternative measures of frangiblebehavior and the frangibility index such that a glass article fallingwithin one criteria level will be assigned the corresponding degree offrangibility and frangibility index. If the frangibility index based onany of the four criteria listed in Table 1 is 3 or greater, the glassarticle is classified as frangible.

Applying the foregoing frangibility index to the samples shown in FIGS.13a and 13b , glass plate a fragmented into multiple ejected smallpieces and exhibited a large degree of crack branching from the initialcrack to produce the small pieces. Approximately 50% of the fragmentsare less than 1 mm in size and it is estimated that about 8 to 10 cracksbranched from the initial crack. Based upon the criteria listed in Table1, glass plate a has a frangibility index of between about 4-5, and isclassified as having a medium-high degree of frangibility.

A glass article having a frangibility index of less than 3 (lowfrangibility) may be considered to be non-frangible or substantiallynon-frangible. Glass plates b, c, and d each lack fragments having adiameter of less than 1 mm, multiple branching from the initial crackformed upon impact and fragments ejected more than 5 cm from theiroriginal position. Glass plates b, c, and d are non-frangible and thushave a frangibility index of 1 (not frangible).

As previously discussed, the observed differences in behavior betweenglass plate a, which exhibited frangible behavior, and glass plates b,c, and d, which exhibited non-frangible behavior, in FIGS. 4a and 4b canbe attributed to differences in central tension CT among the samplestested. The possibility of such frangible behavior is one considerationin designing various glass products, such as cover plates or windows forportable or mobile electronic devices such as cellular phones,entertainment devices, and the like, as well as for displays forinformation terminal (IT) devices, such as laptop computers. Moreover,the depth of the compression layer DOL and the maximum value ofcompressive stress CS that can be designed into or provided to a glassarticle are limited by such frangible behavior.

Accordingly, the strengthened glass articles described herein, in someembodiments, exhibit a frangibility index of less than 3 when subjectedto a point impact sufficient to break the strengthened glass article. Inother embodiments, non-frangible strengthened glass articles may achievea frangibility index of less than 2 or less than 1.

The strengthened glass articles described herein demonstrate improvedfracture resistance when subjected to repeated drop tests. The purposeof such drop tests is to characterize the performance of such glassarticles in normal use as display windows or cover plates for handheldelectronic devices such as cell phones, smart phones, and the like.

A typical ball drop test concept that is currently in use is shown inFIG. 5a . The ball drop test assembly 250 includes a solid, hardsubstrate 212 such as a granite slab or the like and a steel ball 230 ofpredetermined mass and diameter. A glass sample 220 is secured to thesubstrate 212, and a piece of sandpaper 214 having the desired grit isplaced on the upper surface of the glass sample 220 opposite thesubstrate 212. The sandpaper 214 is placed on the glass sample 220 suchthat the roughened surface 214 a of the sandpaper contacts the uppersurface 222 of the glass sample 220. The steel ball 230 is allowed tofall freely from a predetermined height h onto the sandpaper 214. Theupper surface 222 or compression face of the glass sample 220 makescontact with the roughened surface 214 a of the sandpaper 214,introducing cracks into the surface of the upper surface/compressionface 222. The height h may be increased incrementally until either amaximum height is reached or the glass sample fractures.

The ball drop test 250 described hereinabove does not represent the truebehavior of glass when dropped onto and contacted by a rough surface.Instead, it is known that the face of the glass bends outward intension, rather than inward in compression as shown in FIG. 5 a.

An inverted ball on sandpaper (IBoS) test is a dynamic component leveltest that mimics the dominant mechanism for failure due to damageintroduction plus bending that typically occurs in strengthened glassarticles that are used in mobile or hand held electronic devices, asschematically shown in FIG. 5c . In the field, damage introduction (a inFIG. 5c ) occurs on the top surface of the glass. Fracture initiates onthe top surface of the glass and damage either penetrates thecompressive layer (b in FIG. 5c ) or the fracture propagates frombending on the top surface or from center tension (c in FIG. 5c ). TheIBoS test is designed to simultaneously introduce damage to the surfaceof the glass and apply bending under dynamic load.

An IBoS test apparatus is schematically shown in FIG. 5b . Apparatus 200includes a test stand 210 and a ball 230. Ball 230 is a rigid or solidball such as, for example, a stainless steel ball, or the like. In oneembodiment, ball 230 is a 4.2 gram stainless steel ball having diameterof 10 mm. The ball 230 is dropped directly onto the glass sample 218from a predetermined height h. Test stand 210 includes a solid base 212comprising a hard, rigid material such as granite or the like. A sheet214 having an abrasive material disposed on a surface is placed on theupper surface of the solid base 212 such that surface with the abrasivematerial faces upward. In some embodiments, sheet 214 is sandpaperhaving a 30 grit surface and, in other embodiments, a 180 grit surface.Glass sample 218 is held in place above sheet 214 by sample holder 215such that an air gap 216 exists between glass sample 218 and sheet 214.The air gap 216 between sheet 214 and glass sample 218 allows the glasssample 218 to bend upon impact by ball 230 and onto the abrasive surfaceof sheet 214. In one embodiment, the glass sample 218 is clamped acrossall corners to keep bending contained only to the point of ball impactand to ensure repeatability. In some embodiments, sample holder 214 andtest stand 210 are adapted to accommodate sample thicknesses of up toabout 2 mm. The air gap 216 is in a range from about 50 μm to about 100μm. An adhesive tape 220 may be used to cover the upper surface of theglass sample to collect fragments in the event of fracture of the glasssample 218 upon impact of ball 230.

Various materials may be used as the abrasive surface. In a oneparticular embodiment, the abrasive surface is sandpaper, such assilicon carbide or alumina sandpaper, engineered sandpaper, or anyabrasive material known to those of ordinary skill in the art for havingcomparable hardness and/or sharpness. In some embodiments, sandpaperhaving 30 grit, as it has a known range of particle sharpness, a surfacetopography more consistent than concrete or asphalt, and a particle sizeand sharpness that produces the desired level of specimen surfacedamage.

In one aspect, a method 300 of conducting the IBoS test using theapparatus 200 described hereinabove is shown in FIG. 5d . In Step 310, aglass sample (218 in FIG. 5d ) is placed in the test stand 210,described previously and secured in sample holder 215 such that an airgap 216 is formed between the glass sample 218 and sheet 214 with anabrasive surface. Method 300 presumes that the sheet 214 with anabrasive surface has already been placed in test stand 210. In someembodiments, however, the method may include placing sheet 214 in teststand 210 such that the surface with abrasive material faces upward. Insome embodiments (Step 310 a), an adhesive tape 220 is applied to theupper surface of the glass sample 218 prior to securing the glass sample218 in the sample holder 210.

In Step 320, a solid ball 230 of predetermined mass and size is droppedfrom a predetermined height h onto the upper surface of the glass sample218, such that the ball 230 impacts the upper surface (or adhesive tape220 affixed to the upper surface) at approximately the center (i.e.,within 1 mm, or within 3 mm, or within 5 mm, or within 10 mm of thecenter) of the upper surface. Following impact in Step 320, the extentof damage to the glass sample 218 is determined (Step 330). Aspreviously described hereinabove, herein, the term “fracture” means thata crack propagates across the entire thickness and/or entire surface ofa substrate when the substrate is dropped or impacted by an object.

In test method 300, the sheet 218 with the abrasive surface may bereplaced after each drop to avoid “aging” effects that have beenobserved in repeated use of other types (e.g., concrete or asphalt) ofdrop test surfaces.

Various predetermined drop heights h and increments are typically usedin test method 300. The test may, for example, utilize a minimum dropheight to start (e.g., about 10-20 cm). The height may then be increasedfor successive drops by either a set increment or variable increments.The test 300 is stopped once the glass sample 218 breaks or fractures(Step 331). Alternatively, if the drop height h reaches the maximum dropheight (e.g., about 80 cm) without glass fracture, the drop test method300 may also be stopped, or Step 320 may be repeated at the maximumheight until fracture occurs.

In some embodiments, the IBoS test method 300 is performed only once oneach glass sample 218 at each predetermined height h. In otherembodiments, however, each sample may be subjected to multiple tests ateach height.

If fracture of the glass sample 218 has occurred (Step 331 in FIG. 15d), the IBoS test 300 is ended (Step 340). If no fracture resulting fromthe ball drop at the predetermined drop height is observed (Step 332),the drop height is increased by a predetermined increment (Step334)—such as, for example 5, 10, or 20 cm—and Steps 320 and 330 arerepeated until either sample fracture is observed (331) or the maximumtest height is reached (336) without sample fracture. When either Step331 or 336 is reached, the test method 300 is ended.

When the ball is dropped onto the surface of the glass from a height of100 cm, the damage resistance of the strengthened glasses describedhereinabove may be expressed in terms of a “survival rate” whensubjected to the inverted ball on sandpaper (IBoS) test described above.For example, a strengthened glass article is described as having a 60%survival rate when dropped from a given height when three of fiveidentical (or nearly identical) samples (i.e., having approximately thesame composition and, when strengthened, approximately the same CS andDOC or DOL) survive the IBoS drop test without fracture.

To determine the survivability rate of the strengthened glass articlewhen dropped from a predetermined height using the IBoS test method andapparatus described hereinabove, at least five identical (or nearlyidentical) samples (i.e., having approximately the same composition andapproximately the same CS and DOC or DOL) of the strengthened glass aretested, although larger numbers (e.g., 10, 20, 30, etc.) of samples maybe subjected to testing to raise the confidence level of the testresults. Each sample is dropped a single time from the predeterminedheight (e.g., 80 cm) and visually (i.e., with the naked eye) examinedfor evidence of fracture (crack formation and propagation across theentire thickness and/or entire surface of a sample. A sample is deemedto have “survived” the drop test if no fracture is observed after beingdropped. The survivability rate is determined to be the percentage ofthe sample population that survived the drop test. For example, if 7samples out of a group of 10 did not fracture when dropped from thepredetermined height, the survivability rate of the glass would be 70%.

The strengthened glass articles described herein also demonstrateimproved surface strength when subjected to abraded ring-on-ring (AROR)testing. The strength of a material is defined as the stress at whichfracture occurs. The abraded ring-on-ring test is a surface strengthmeasurement for testing flat glass specimens, and ASTM C1499-09(2013),entitled “Standard Test Method for Monotonic Equibiaxial FlexuralStrength of Advanced Ceramics at Ambient Temperature,” serves as thebasis for the ring-on-ring abraded ROR test methodology describedherein. The contents of ASTM C1499-09 are incorporated herein byreference in their entirety. In one embodiment, the glass specimen isabraded prior to ring on ring testing with 90 grit silicon carbide (SiC)particles that are delivered to the glass sample using the method andapparatus described in Annex A2, entitled “abrasion Procedures,” of ASTMC158-02(2012), entitled “Standard Test Methods for Strength of Glass byFlexure (Determination of Modulus of Rupture). The contents of ASTMC158-02 and the contents of Annex 2 in particular are incorporatedherein by reference in their entirety.

Prior to ring-on-ring testing a surface of the glass sample is abradedas described in ASTM C158-02, Annex 2, to normalize and/or control thesurface defect condition of the sample using the apparatus shown inFigure A2.1 of ASTM C158-02. The abrasive material is sandblasted ontothe sample surface at a load of 15 psi using an air pressure of 304 kPa(44 psi). After air flow is established, 5 cm³ of abrasive material isdumped into a funnel and the sample is sandblasted for 5 seconds afterintroduction of the abrasive material.

For the ring-on-ring test, a glass specimen having at least one abradedsurface 412 is placed between two concentric rings of differing size todetermine equibiaxial flexural strength (i.e., the maximum stress that amaterial is capable of sustaining when subjected to flexure between twoconcentric rings), as schematically shown in FIG. 19. In the abradedring-on-ring configuration 400, the abraded glass specimen 410 issupported by a support ring 420 having a diameter D₂. A force F isapplied by a load cell (not shown) to the surface of the glass specimenby a loading ring 430 having a diameter D₁.

The ratio of diameters of the loading ring and support ring D₁/D₂ may bein a range from about 0.2 to about 0.5. In some embodiments, D₁/D₂ isabout 0.5. Loading and support rings 430, 420 should be alignedconcentrically to within 0.5% of support ring diameter D₂. The load cellused for testing should be accurate to within ±1% at any load within aselected range. In some embodiments, testing is carried out at atemperature of 23±2° C. and a relative humidity of 40±10%.

For fixture design, the radius r of the protruding surface of theloading ring 430, h/2≤r≤3h/2, where h is the thickness of specimen 410.Loading and support rings 430, 420 are typically made of hardened steelwith hardness HR_(c)>40. ROR fixtures are commercially available.

The intended failure mechanism for the ROR test is to observe fractureof the specimen 410 originating from the surface 430 a within theloading ring 430. Failures that occur outside of this region—i.e.,between the loading rings 430 and support rings 420—are omitted fromdata analysis. Due to the thinness and high strength of the glassspecimen 410, however, large deflections that exceed ½ of the specimenthickness h are sometimes observed. It is therefore not uncommon toobserve a high percentage of failures originating from underneath theloading ring 430. Stress cannot be accurately calculated withoutknowledge of stress development both inside and under the ring(collected via strain gauge analysis) and the origin of failure in eachspecimen. AROR testing therefore focuses on peak load at failure as themeasured response.

The strength of glass depends on the presence of surface flaws. However,the likelihood of a flaw of a given size being present cannot beprecisely predicted, as the strength of glass is statistical in nature.A Weibull probability distribution is therefore generally used as astatistical representation of the data obtained.

While typical embodiments have been set forth for the purpose ofillustration, the foregoing description should not be deemed to be alimitation on the scope of the disclosure or appended claims.Accordingly, various modifications, adaptations, and alternatives mayoccur to one skilled in the art without departing from the spirit andscope of the present disclosure or appended claims.

1. A glass article, the glass article comprising a thickness tin a rangefrom 0.1 mm up to 1.0 mm and a compressive region having a compressivestress CS_(s) in a range from 400 MPa to 1200 MPa at a surface of theglass article, wherein the compressive region extends from the surfaceto a depth of compression DOC, wherein 0.1t≤DOC≤0.25t, and has acompressive stress profile, the compressive stress profile comprising:a. a first portion b extending from the surface to a first depth belowthe surface and having a slope m_(b), wherein −40 MPa/μm≥m_(b)≥−200MPa/μm, wherein the first depth is in a range from 3 μm to 15 μm; and b.a second portion c extending from a second depth below the surface tothe depth of compression DOC and having a slope m_(c), wherein −0.4MPa/μm≥m_(c)≥−3.0 MPa/μm, wherein the second depth is in a range from 3μm to 15 μm.
 2. The glass article of claim 1, wherein the depth ofcompression DOC is in a range from 95 μm to 250 μm.
 3. The glass articleof claim 2, wherein the depth of compression DOC is in a range from 100μm to 190 μm.
 4. The glass article of claim 1, wherein 0.12t≤DOC≤0.22t.5. The glass article of claim 1, wherein the slope m_(c) is in a rangefrom −0.7 MPa/μm to −2.7 MPa/μm.
 6. The glass article of claim 5,wherein the slope m_(c) is in a range from −1.5 MPa/μm to −2.7 MPa/μm.7. The glass article of claim 1, wherein the glass article comprises analkali aluminosilicate glass.
 8. The glass article of claim 7, whereinthe alkali aluminosilicate glass comprises up to 10 mol % Li₂O.
 9. Theglass article of claim 7, wherein the alkali aluminosilicate glasscomprises at least 4 mol % P₂O₅ and from 0 mol % to 4 mol % B₂O₃,wherein 1.3<[(P₂O₅+R₂O)/M₂O₃]<2.3, where M₂O₃=Al₂O₃+B₂O₃, and R₂O is thesum of monovalent cation oxides present in the alkali aluminosilicateglass.
 10. The glass article of claim 7, wherein the glass islithium-free.
 11. The glass article of claim 7, wherein the glassconsists essentially of from 40 mol % to 70 mol % SiO₂; from 4 mol % to15 mol % P₂O₅; from 13 to 30 mol % R_(x)O, wherein R_(x)O is the sum ofthe alkali metal oxides, alkaline earth metal oxides, and transitionmetal monoxides present in the glass, wherein R_(x)O comprises from 13mol % to 25 mol % Na₂O and from 0 mol % to 1 mol % K₂O, from 11 to 29mol % M₂O₃, where M₂O₃=Al₂O₃+B₂O₃, wherein M₂O₃ comprises from 11 mol %to 25 mol % Al₂O₃, and from 0 mol % to 4 mol % B₂O₃; and 3 mol % or lessof one or more of TiO₂, MnO, Nb₂O₅, MoO₃, Ta₂O₅, WO₃, ZrO₂, Y₂O₃, La₂O₃,HfO₂, CdO, SnO₂, Fe₂O₃, CeO₂, As₂O₃, Sb₂O₃, Cl, and Br; and1.3<[(P₂O₅+R₂O)/M₂O₃]≤2.3, where R₂O is the sum of monovalent cationoxides present in the glass.
 12. An electronic device comprising theglass article of claim 1.